Microaneurysms were counted separately in a masked manner on stereo colour fundus photographs and on stereo fluorescein angiograms of 100 patients with unselected forms of diabetic retinopathy. Serial studies obtained 6 months apart were available for most patients, and a total of 315 separate photographic studies were evaluated. In addition, 118 photographic studies of 38 young patients with type I diabetes mellitus were evaluated. About twice as many microaneurysms were detected on the fluorescein angiograms as on the colour photographs for both groups of patients (p less than 0.05). Fluorescein angiography showed microaneurysms in 57% of the eyes in the second group photographed for the first time that had no detectable microaneurysms on colour photography. The use of fluorescein angiography is recommended in diabetic patients when detection of subtle microvascular abnormalities is deemed essential.